The interaction of insulin and epidermal growth factor (EGF) with cultured human breast cancer cells has been investigated. All cell lines examined have specific high affinity receptors for insulin and EGF. However, only certain of these cell lines have retained the ability to respond biologically to these hormones. In these lines both insulin and EGF in physiologic concentrations stimulate protein, RNA, and DNA synthesis in a sequential fashion. Cell proliferation is also significantly augmented by these factors although the effects at optimal concentrations are not additive suggesting similar pathways of action. These cell lines are currently being used to study the biochemical steps of the mechanisms of insulin and EGF action distal to receptor binding.